As is well known, the core network of 3G (The Third Generation Mobile Communication Technology) is composed of a MSC (Mobile Switching Center), a VLR (Visitor Location Register), a HLR (Home Location Register) and a RNC (Radio Network Controller), wherein MSC can be divided according to functions into GMSC (Gateway Mobile Switching Center), which is a MSC responsible for inquiring the routing information of the called subscriber, and VMSC (Visitor Mobile Switching Center), which is a visited MSC of the subscriber and also a destination to be routed by a call; VLR is a visitor location register of a subscriber, HLR is a home location register of a subscriber, and RNC is a network element responsible for implementing the function of subscriber wireless access.
Pre-paging means that a GMSC will firstly send a Send Routing Information (SRI) message to a HLR when inquiring routing, and then the HLR will send a Provide Roaming Number (PRN) message to a VLR where the called subscriber is located; after receiving the PRN message, the VLR where the called subscriber is located will check the parameters in the message, and will return a PRN RSP (response) message to the HLR after pre-paging is over if the message requests the VLR where the called subscriber is located to start a pre-paging procedure, and the HLR will return a SRI RSP message to the GMSC; at this moment, the procedure of pre-paging is completely finished.
Roaming Retry is a technique adopted to solve the problem of call failure that occurs when a terminal subscriber is called while the subscriber moves out from the current location area; since it may take a period of time from when the HLR acquires the roaming number by inquiring the VMSC of the called subscriber to when the VMSC where the subscriber is located receives an IAM (Initial Address Message) sent by the GMSC, a subscriber which is of high mobility and meanwhile is on the border of a location area may move into another location area; this technique is favourable to improving probability of call success for a subscriber of high mobility.
The flow of the existing roaming retry technique is as shown in FIG. 1:
Step 101: firstly a GMSC sending a SRI message carrying a Roaming Retry Support identifier to a HLR;
Step 102: then the HLR forwarding a PRN message carrying a Roaming Retry Support identifier to an Old VMSC/VLR for requesting for a roaming number;
The above SRI and PRN messages include a pre-paging support indication field;
Step 103: the Old VMSC/VLR returning a PRN ACK message carrying a Mobile Station Roaming Number (MSRN) to the HLR;
Step 104: the HLR returning a SRI ACK message carrying the MSRN to the GMSC;
Step 105: the GMSC sending an Initial Address Message (IAM) to the Old VMSC;
Step 106: after receiving the IAM message, the Old VMSC/VLR initiating a paging;
Step 107: the subscriber moving at this moment, initiating a procedure for location update under a new VMSC, and a MS (Mobile Station) sending a Location Update request to the new VMSC/VLR;
Step 108: the new VMSC sending an update location request to the HLR;
Step 109: at this moment, the HLR sending a Cancel Location message to the Old VMSC, then the Old VMSC returning a Cancel Location Ack to the HLR.
Step 110: upon receiving the Cancel Location message, the Old VMSC firstly stopping a paging timer to cancel paging since paging is still performed at this moment, and notifying the GMSC to perform roaming retry through step 111; meanwhile the new VMSC/VLR starting the procedure of inserting subscriber data;
Step 111: the Old VMSC/VLR sending a RCH (Resume Call Handling) message carrying a roaming retry indication identifier to notify the GMSC to perform roaming retry;
Step 112: the GMSC releasing the call to the Old VMSC/VLR using a REL/RLC message (a releasing message of ISUP, corresponding to the IAM message);
Step 113: after receiving the RCH message, the GMSC initiating a procedure of obtaining the roaming number for the second time at this moment, which is called as roaming retry, wherein the SRI message sent for the second time does not include Roaming Retry Support identifier any more;
Step 114: after completing location update, the HLR postponing sending the PRN to the new VMSC/VLR;
Step 115: after receiving the PRN ACK sent by the NEW VMSC/VLR, the HLR returning a SRI ACK message containing MSRN'(a new mobile station roaming number) to the GMSC;
Step 116: the GMSC sending an Initial Address Message (IAM) to the NEW VMSC/VLR using this MSRN';
Step 117: after completing location update, the NEW VMSC/VLR initiating a paging procedure.
Processing of normal MT (Mobile Terminated) call procedure will be performed subsequently.
As can be seen from the flow and various protocol files described specifically above, in the current method for triggering roaming retry, if the subscriber starts pre-paging when the Old MSC/VLR receives SRI/PRN for the first time, the PRN procedure will fail directly if processing is carried out according to the previous protocol when the subscriber moves during pre-paging and receives a Cancel Location message; in step 103, a PRN ACK message carrying an error indication is sent to the HLR, and roaming retry is not taken into consideration if a Cancel Location is received during pre-paging according to the SDL (Specification and Description Language) chart of a 3GPP improved protocol, and actually the processing is still performed according to call failure at this moment.
Therefore, the prior art only provides a solving method for the subscribers of high mobility in a non pre-paging scene, and processing is still performed according to call failure for the subscribers of pre-paging.